In a single beam satellite system, all the transponders operate through the same antenna beam, so that an earth station anywhere in the coverage area of the beam can communicate with another earth station anywhere else in the coverage area. This includes satellites where a single beam is constructed by combining separate geographic coverage area beams into a functionally single beam which operates through all or a group of transponders on the satellite. In a spot beam satellite system, either one transponder or a group of transponders operate in conjunction with receive and transmit antenna beams that have a coverage area that is geographically distinct from the coverage areas of other receive and transmit antenna beams operating with other transponders or groups of transponders.
Using multiple spot beams to provide communications over a large geographic area instead of using a large single beam has certain well-known advantages. However, it is generally necessary to provide full interconnectivity from any part of the overall coverage area to any other part, i.e., from any one spot beam coverage area to any other. This interconnectivity can be achieved by providing a suitably large number of transponders on the satellite. In a spot beam system with two spot beams, for example, four conventional transponders are necessary to achieve full two-way interconnectivity between the beams. Each path from beam to beam requires a separate transponder as shown in FIG. 3 and as follows:
______________________________________ From To Transponder ______________________________________ Beam A Beam A No. 1 Beam A Beam B No. 2 Beam B Beam A No. 3 Beam B Beam B No. 4 ______________________________________
For a coverage area with only two spot beams, the need for separate transponders for each possible route from beam to beam is not too onerous to implement. However with a larger number of spot beams, the number of transponders required for full interconnectivity increases dramatically as shown below.
______________________________________ Number of Spot Beams Number of Transponders ______________________________________ 2 4 4 16 6 36 8 64 16 256 ______________________________________
The increasingly large number of transponders quickly exceeds the capabilities of a single satellite. While several satellites could be co-located to accommodate such a large number of transponders, there would not be complete interconnection of beams until all the satellites were operational, and continuity would be lost in the event of a satellite failure. The use of separate transponders for each path among the spot beams, therefore, is not a very practical approach. However, satellite systems employing many small spot beams would be attractive economically and operationally if the problems created by a large number of transponders could be avoided. The commonly accepted conclusion is that this problem can be solved only by demodulation, switching and remodulation of signals on-board the satellite. However, such techniques are heavy, expensive and not yet proven technologically for most commercial applications.
With on-board switching, signals from each uplink are received, demodulated, and then switched to the appropriate downlink beam for modulation and transmission to the correct spot beam coverage area. This reduces the number of transponders required for "N" spot beams from N.sup.2, as required without switching, to N. However, because of the weight, cost, and complexity of this active switching arrangement, it would be useful to devise a practical system in which the switching occurs without on-board demodulation so that the satellite is passive rather than an active switching device.
Thus, to have full interconnectivity of separate spot beams, the number of transponders will be very large when there are any significant number of spot beams or there must be complicated on-board demodulation and switching.
For purposes of this application, the following terms are used as indicated below.
"Beam" is used to refer to a spot beam covering a portion of the total satellite coverage area or to a larger beam covering the same area as a number of spot beams up to and including the entire coverage area of the satellite. If differentiation between a spot beam and a larger beam is important to the discussion, it will be clear from the context which is meant. Where beams are intended to be functionally independent so that there will be no interference among beam signals, several means of separation can be used including geographic separation of coverage areas, disjoint frequency utilization, and orthogonal wave polarization. In the discussion and figures in this application it is assumed that spot beams are kept separate by one or more of these techniques unless otherwise indicated.
"Route" as a noun is used to refer to an electronic path through a satellite or to an electronic path from one earth station to another.
"Routing" as a verb is used to refer to the selection of signal parameters such as frequency, polarization, and timing such that a signal is made to follow an appropriate route through a satellite or from one earth station to another.
"Switching" is used to refer to the selection of signal parameters for routing together with necessary commands to earth stations so that a communications route is established for a period of time between or among desired earth stations.
"Transponder" is used to refer to the equipment that typically constitutes a single path, functionally separate from all other paths, for electronic signals through a satellite.